Specify the default VOL connector to use with an environment variable.

This implicitly adds support for changing the VOL connector for command-line
tools or any application linked with the library.

Also, add 'make check-vol' support for all directories, clearing up necessary
issues in testing scripts, etc.

testpar/Makefile.am

@@ -42,6 +42,11 @@ testphdf5_SOURCES=testphdf5.c t_dset.c t_file.c t_file_image.c t_mdset.c \
 # The tests all depend on the hdf5 library and the test library
 LDADD = $(LIBH5TEST) $(LIBHDF5)
 
+# Test with just the native connector, with a single pass-through connector
+# and with a doubly-stacked pass-through.
+VOL_LIST = native "pass_through under_vol=0;under_info={}" \
+           "pass_through under_vol=505;under_info={under_vol=0;under_info={}}"
+
 # Temporary files
 # MPItest.h5 is from t_mpi
 # Para*.h5 are from testphdf

testpar/t_cache.c

@@ -24,6 +24,7 @@
 
 #include "H5ACpkg.h"
 #include "H5Cpkg.h"
+#include "H5CXprivate.h"
 #include "H5Fpkg.h"
 #include "H5Iprivate.h"
 #include "H5MFprivate.h"